This invention relates to molecular beam epitaxy (MBE) and, more particularly, to the MBE growth of Sn-doped Group III(a)-V(a) Ga-containing layers with abrupt doping profiles.
Many devices, such as FETs, IMPATTs, varactors, and microwave mixer diodes require precise control of layer thickness in the sub-micron range and close tolerance in carrier density of a predetermined doping profile. Such applications may require an abrupt change in doping concentration within a thin layer, typically only 1,000 A thick. MBE has demonstrated the ability to achieve reproducibly layers of Group III(a)-V(a) compounds, most noteably GaAs, which are extremely smooth, ultra thin and have controllable doping profiles.
The basic MBE process is described by J. R. Arthur, Jr. in U.S. Pat. No. 3,615,931 issued on Oct. 26, 1971. Precise control of layer thickness in the sub-micron range is accomplished by the reduction of the growth rate and the ability to start and stop growth virtually instantaneously. The growth rate is governed primarily by the effusion cell temperature (i.e., the evaporation rate of Ga in the case of growing GaAs), and the sharpness of the interface between layers is governed by a mechanical shutter in front of the effusion cells which is used to interrupt the molecular beam abruptly. The nominal growth rate of MBE GaAs, for example, is about 1 .mu.m/hr. With growth rates of this order, the shutter time is much less than the time for the growth of a monolayer. Since the epitaxial growth temperature for MBE is relatively low (450.degree. to 650.degree.C), as compared to liquid phase epitaxy (800.degree.C) or the chemical vapor deposition (750.degree.C), abrupt interfaces can be realized if diffusion and surface segregation are negligible.
From the standpoint of doping, it is pointed out in U.S. Pat. No. 3,751,310 issued on Aug. 7, 1973, that reliance on prior semiconductor technology in the determination of an appropriate dopant for MBE is generally misplaced. That patent explains for example that Zn, a common p-type dopant in other GaAs fabrication techniques, is unsuitable for MBE because of its low sticking coefficient at the usual growth temperatures. On the other hand, Sn and Si are identified as n-type MBE dopants whereas Ge is amphoteric depending on whether the surface structure is Ga-stabilized (p-type) or As-stabilized (n-type). Aside from Ge, magnesium is also a suitable MBE p-type dopant. Its sticking coefficient, which is relatively low on GaAs, increases dramatically if Al is incorporated in the layer to form AlGaAs (see, U.S. Pat. No. 3,829,084 granted to M. B. Panish and myself on Oct. 1, 1974).